The objectives of this study are to characterize possible age-related differences in the in vitro responses of murine lymphohemopoietic cells to mitogen and their differential susceptibility to freezing damage. Murine splenic lymphocytes were subjected to a broad range of cooling rates using a microcomputer controlled rate controller, held at -196 degrees C, rapidly thawed, and tested for functional activity. The incorporation of 3H-thymidine was used to assess cellular proliferation after in vitro activation of the lymphocytes by various mitogens. A high percentage of functionally unimpaired cells was recovered at cooling rates from -0.25 degrees C to -5.0 degrees C/min., a broader range of rates than previously reported for murine and human lymphocytes.